How are INSOLUBLE impurities removed during recrystallization? How are SOLUBLE impurities removed?
thank you.
If I understand the question, for insoluble materials add enough solvent to dissolve the major material you want to keep but doesn't dissolve the insoluble material, filter, then recrystallize the material from solvent.
Soluble materials are removed by choosing solvents in which the solubility of the impurity is different than the solubility of the material you want to keep. Through several recrystallizations the soluble materials can be removed at the expense of leaving some of the material you want to keep behind in the solvent.
During recrystallization, insoluble impurities can be removed by several methods:
1. Filtration: After the recrystallization process, the mixture is cooled, causing the desired compound to precipitate as crystals. Insoluble impurities remain as solid particles in the solution. By using a filtration setup (such as a Buchner funnel and filter paper), the crystals can be separated from the solution while retaining the insoluble impurities on the filter paper.
2. Decantation: In some cases, the insoluble impurities settle at the bottom of the container due to their higher density. By carefully pouring off the liquid (supernatant) and leaving behind the settled impurities, separation can be achieved.
On the other hand, soluble impurities require different techniques for their removal:
1. Hot Filtration: Similar to the filtration process for insoluble impurities, hot filtration is performed when the compound of interest and the soluble impurities are dissolved in a hot solvent. The hot solution is filtered to remove any insoluble impurities. As the solution cools, the compound of interest will crystallize, leaving the soluble impurities behind in the filtrate.
2. Washing: By using a suitable solvent, insoluble impurities that adhere to the crystals can be removed through a process called washing. The solvent is added to the crystals, and the mixture is briefly stirred before being filtered again. This helps in dissolving the impurities and washing them away, leaving behind purer crystals.
It's important to note that the specific method used to remove impurities during recrystallization may vary depending on the nature of the impurities and the compound being purified. Efficient recrystallization often involves a combination of these techniques to obtain pure crystals.